A solid polymer fuel cell is a system for taking out electric power by using, as a fuel, pure hydrogen, hydrogen gas obtained by modifying alcohol, etc. and electrochemically controlling the reaction between the hydrogen and the oxygen in the air.
A solid polymer fuel cell uses a solid hydrogen ion selective transmission type organic film as an electrolyte, so compared with conventional alkali type fuel cells, phosphoric acid type fuel cells, molten carbonate type fuel cells, solid electrolyte type fuel cells, and other such fuel cells which use electrolytes comprised of aqueous solution type electrolytes or molten salt type electrolytes and other fluid media, greater compactness becomes possible. Development efforts are underway for application to electric vehicles etc.
The configuration of a typical solid polymer fuel cell is shown in FIG. 1. The solid polymer fuel cell 1 is comprised of a solid polymer film 2 for forming an electrolyte, catalyst electrode parts (3a, 3b) comprised of carbon fine particles and precious metal ultra fine particles provided on the two surfaces of this solid polymer film 2, current collectors comprised of felt-like carbon fiber aggregates which have the functions of taking out electrons produced at the anode side catalyst electrode part 3a and feeding the reaction gas of oxygen-based gas or hydrogen-based gas to the catalyst electrode parts (3a, 3b) (usually called “carbon paper” (4a, 4b)), and separators (5a, 5b) which receive current from the carbon paper (4a, 4b) and separate the oxygen-based gas and hydrogen-based gas, all stacked together.
The basic principle of a solid polymer fuel cell 1 is as follows: That is, in a solid polymer fuel cell 1, the fuel of hydrogen gas (H2) 8 is supplied from the anode side, passes through the gas diffusion layer of the carbon paper 4a, and, at the anode side 6 catalyst electrode part 3a, breaks down into hydrogen ions (H+) and electrons (e−) by the reaction of H2→2H++2e−. The hydrogen ions (H+) pass through the electrolyte of the solid polymer membrane 2 and reach the cathode side 7 catalyst electrode part 3b. 
On the other hand, the electrons (e−) 10 pass from the anode side carbon paper 4a through the anode side separator 5a and conductor 16 to reach the cathode side separator 5b and further pass through the cathode side carbon paper 4b to reach the cathode side catalyst electrode part 3b. At the cathode side catalyst electrode part 3b, the hydrogen ions (H+) which have arrived through the solid polymer film 2 and the electrons (e−) which have arrived through the conductor 16 react with the oxygen (O2) in the air 9 which is fed from the cathode side 7 (2H++2e−+l/2O2→H2O) whereby water (H2O) is produced. The produced water (H2O) moves through the cathode side carbon paper 4b to the cathode side separator 5b. At the time of this reaction, the electrons (e−) 10 which were produced at the anode side 6 catalyst electrode part 3a pass through the carbon paper 4a and from the anode side 6 separator 5a through the conductor 16 to flow to the cathode side 7 separator 5b whereby current and voltage are generated across the electrodes of the cathode side and anode side of the catalyst electrode parts 3.
The solid polymer film 2 is comprised of an electrolyte having a strong acidity immobilized in a film and functions as an electrolyte passing hydrogen ions (H+) by control of the dew point inside of the battery.
The component member separator 5 of the solid polymer fuel cell 1 has the role of separating the two types of reaction gases, that is, the cathode side 7 air 9 and the anode side 6 hydrogen gas 8, and providing flow paths for supplying these reaction gases and the role of discharging the water produced by the reaction from the cathode side 7 when stacking basic units of the solid polymer fuel cell shown in FIG. 1.
Further, in general, the solid polymer fuel cell 1 uses a solid polymer membrane made of an electrolyte exhibiting a strong acidity. Due to the reaction, it operates at a temperature of about 150° C. or less and generates water. For this reason, the separator 5 for solid polymer fuel cell use is required to have, as material properties, corrosion resistance and durability and is required to have good electroconductivity for efficient conduction of current through the carbon paper 4 and low contact resistance with carbon paper.
In the past, as the material for the separator for a solid polymer fuel cell, much use has been made of carbon-based materials. However, separators made of carbon-based materials cannot be made thin due to problems of brittleness and therefore obstruct increased compactness. In recent years, breakage-resistant separators made of carbon-based materials have also been developed, but they are expensive in cost, so are disadvantageous economically.
On the other hand, separators using metal materials are free from problems of brittleness compared with carbon-based materials, so in particular enable increased compactness of solid polymer fuel cell systems. Separators using the low cost material stainless steel or titanium alloy or other metal materials are being developed. Numerous ones have been proposed (for example, see PLTs 1, 2, and 12 to 20).
However, separators made of stainless steel or separators made of titanium or titanium alloy become larger in contact resistance with the carbon paper due to the passivation film formed on the surfaces, so had the problem of greatly reducing the energy efficiency of the fuel cells.
For this reason, numerous methods for reducing the contact resistance between member surfaces and carbon paper have been proposed for stainless-steel separators and titanium and titanium-alloy separators in the past.
For example, separator materials for solid polymer fuel cell use have been proposed using the methods of forming on the surface of stainless steel (SUS304) a large number of protruding shapes by press forming and forming on the end faces of the front end sides a predetermined thickness of a gold plating layer (for example, see PLT 3), depositing on a stainless steel or titanium surface a precious metal or a precious metal alloy to thereby lower the contact resistance with carbon paper (for example, see PLT 4), etc. However, these methods require that the stainless steel or titanium surface be treated to form a gold plating or other expensive precious metal layer for imparting conductivity, so had the problem of an increased cost of production of the separator.
On the other hand, various methods have been proposed for reducing the amount of use of expensive precious metals or for reducing the contact resistance between separator member surfaces and carbon paper without using a precious metal.
For example, to reduce the contact resistance between a stainless steel surface and carbon paper, the method of causing the Cr in the stainless steel to precipitate as chromium carbides in the annealing process of stainless steel and using the chromium carbides which are exposed from the passivation film surface which is formed on the stainless steel surface in order to raise the conductivity of the current received from the carbon paper (for example, see PLT 5), and the method of providing the stainless steel surface with a coating film in which SiC, B4C, TiO2, and other conductive compound particles are dispersed, then heating this stainless steel in a nonoxidizing atmosphere at 300 to 1100° C. to break down or consume the main ingredients of the coating film or covering the surface with a carbide-based conductive ceramic to therefore form the conductive compound particles on the stainless steel surface (for example, see PLTs 6 and 7) are known. However, these methods require the step of long heat treatment for forming conductive compounds on the stainless steel surface, so had the problem of a drop in separator productivity or increased manufacturing cost.
Further, in the method of making the Cr in the stainless steel precipitate as chromium carbides in the annealing process, in particular when the annealing time is not sufficient, a chromium-deficient layer forms around the chromium carbides in the steel, a local drop in corrosion resistance is caused in this region, and, when press forming the stainless steel to form the gas flow paths at the separator surface etc., the chromium carbides are liable to act as starting points for cracking of the stainless steel surface.
Further, the method has also been proposed of fastening a carbon layer or carbon particles with good conductivity at the stainless steel surface. For example, the method of forming gas flow paths on a metal sheet by press forming at main parts where the catalyst electrodes are located, then forming a carbon-based conductive coating layer at that surface (for example, see PLT 8), the method of dispersing and press bonding carbon powder to the stainless steel surface to improve the conductivity (for example, see PLT 9), and the method of forming at the stainless steel surface an Ni—Cr-based plating layer or Ta—, Ti—, or Ti—Ta-based plating layer in which carbon-based particles are dispersed (for example, see PLTs 10 and 11) are known. However, with the separators obtained by these methods, due to the pseudo Schottky barriers formed at the carbon side in the electron structure of the interface between the metal and carbon, a large contact resistance is formed at the interface of the stainless steel and carbon layer or carbon particles and as a result the effect of sufficiently reducing the contact resistance with the carbon paper is not obtained.
Further, the method of forming a conductive ceramic layer of one or more of TiN, TiC, CrC, TaC, B4C, SiC, WC, TiN, ZrN, CrN, and HfC at the fuel electrode side feeding hydrogen-based gas at the stainless steel separator (for example, see PLT 21) has been proposed. This method forms a conductive ceramic layer by vapor deposition using a vacuum system etc. or by the dry coating method, but there are limits to the film-forming speed and a drop in yield of the coated substance is forced, so there is the problem of increased manufacturing cost.
Further, the method of affixing hard fine powder having conductivity to the surface of a base material by shot-blasting is also known. For example, a titanium or titanium alloy separator where conductive hard particles of the M23C6 type, M4C type, or MC type which contain a metal element (M) including at least one of chromium, iron, nickel, molybdenum, tungsten, and boron are embedded in a base material surface and dispersed and exposed (for example, see PLT 22) and a stainless steel and stainless steel separator where conductive hard particles of at least one type of carbide-based metal inclusions of the M23C6 type, M4C type, M2C type, and MC type and boride-based metal inclusions of the M2B type which contain a metal element (M) including at least one of chromium, molybdenum, and tungsten are embedded in a base material surface and dispersed and exposed and where a surface roughness is a centerline average roughness Ra of 0.06 to 5 μm (for example, see PLT 23) have been proposed.
Further, a method of shot blasting a separator forming a fuel cell with a solid plating material comprised of core particles which have a higher hardness' than the separator and which are coated with metal having a high corrosion resistance and low contact resistance with carbon so as to make metal coated on the solid plating material forcibly stick to the separator (for example, see PLT 24) or a method using the same technique to embed a very fine amount of a precious metal in stainless steel or titanium or titanium alloy to thereby obtain sufficiently low contact resistance even without coating the entire surface with a precious metal such as with gold plating (for example, see PLT 25) has been proposed.
These methods of affixing hard fine powder having conductivity by shot blasting etc. to the surface of a base material are advantageous methods compared with the methods of heat treatment or vacuum deposition in the point of being methods which do not lower productivity, are low in manufacturing costs, and are simple. On the other hand, with the method of mechanically driving by shot blasting etc. hard conductive particles into the surface of a metal separator base material formed into a desired shape, there is a possibility of strain being introduced into the surface layer part of the base material and the material deforming. Sometimes the flatness of the separator is reduced.
In general, a solid polymer fuel cell has a low output voltage per basic unit of about 1V, so to obtain the desired output, often a large number of fuel cells are stacked and used as a fuel cell stack. Therefore, in the method of affixing hard fine powder having conductivity to the surface of a base material by shot blasting etc., it is necessary to perform the treatment under conditions which minimize warping or distortion of the separators and give separators having a good flatness enabling stacking of fuel cells.
Further, the contact resistance between a separator and carbon paper is preferably as low as possible. For example, the method of depositing a metal having a low contact resistance with carbon of 20 mΩ·cm2 or less at a contact surface pressure of 1 kg·f/cm2 (9.8×104 Pa) on a separator for a fuel cell (for example, see PLT 24) etc. have been proposed.
In the above way, in the past, as the separator base material, the superior corrosion resistance stainless steel and titanium or titanium alloy or other metal materials have been used. To improve the contact resistance between the surface of these separator base materials and carbon paper, metal separators for solid polymer fuel cell use which use various methods to form conductive compound layers on the base material surface or affix conductive compound particles to them have been proposed. From the viewpoint of the contact resistance and flatness demanded from a separator for a solid polymer fuel cell or from the viewpoint of the productivity or manufacturing costs, the results cannot necessarily be said to have been sufficient.
Among these, titanium is gathering attention as a material which is even more superior in corrosion resistance to stainless steel and which has little deterioration even with long term operation. A titanium material for electrode use which comprises titanium plus a platinum group element (Pd, Pt, Ir, Ru, Rh, or Os) to inhibit a drop in conductivity and a method of production of the same (for example, see PLT 26) and a titanium material which comprises a titanium alloy base material containing a precious metal element of the platinum group elements wherein a mixed layer is formed by the precious metal element precipitated at the surface and titanium oxide, the contact resistance is low, and the contact resistance is inhibited from rising over a long period of time (for example, see PLT 27) have been proposed. However, these contain a precious metal, so the material costs become large. Further, in recycling of titanium materials, there was the problem that an added metal element was liable to invite a deterioration in the strength of the titanium, so had to be removed for recycling.